Ion exchange removal of cyanide values

ABSTRACT

A method of removing cyanide values from aqueous solutions containing sodium or potassium cyanide, or the sodium or potassium salts of the complex cyanides of the metals copper, nickel, zinc, silver, gold and cadmium, utilizing a mixed bed of ion exchange resins.

United States Patent Sloan [151 3,656,893 [4 1 Apr. 18, 1972 [541 IONEXCHANGE REMOVAL OF CYANIDE VALUES I [72] Inventor: Walter John Sloan,Newark, Del.

[731 Assignees Wilmington, Del.

22 Filed: July 2,1970

[21] Appl.No.: 52,141

521 u.s.c| ..23/77,23/79,260/2.1-M

[51] Int. Cl ..C01c 3/08, COlc 3/00 [58] Field of Search...

E. I. du' Pont de Nemours and Company,

1,962,559 6/1934 l-lagenestetal. ..23/77 3,262,891 7/1966 Abrams....260/2.1R

Primary Examiner-Oscar R. Vertiz Assistant Examiner-Hoke S. MillerAttorney-Harry J. McCauley [57] ABSTRACT A method of removing cyanidevalues from aqueous solutions containingsodium or potassium cyanide, orthe sodium or potassium salts of the complex cyanides of the metalscopper,

nickel, zinc, silver, gold and cadmium, utilizing a mixed bed of ionexchange resins.

6 Claims, No Drawings 1 ION EXCHANGE REMOVAL OF CYANIDE VALUES BRIEFSUMMARY OF THE INVENTION Generally, this invention comprises the methodof removing cyanide values from an aqueous process solution containing.

.zinc, silver, gold and cadmium, (2) passing the process solution fromstep (1) through an ion exchange bed composed of a mixture of thehydrogen form of a cation exchanger selected from the group consistingof carboxylic, phosphonic, and sulfonic ion exchange resins, and thefree base form of a weak base anion exchange resin to convert a portionof the sodium and potassium salts of the complex cyanides of copper,nickel, zinc, silver, gold and cadmium to their corresponding acids, (3)removing the corresponding acids progressively, as

formed, by combination with the weak baseresin until substantially allcyanide values are removed from the process solution, (4) withdrawingthe process solutionfrom the ion exchange bed and, optionally, (5)removing said acids from the weak base resin of step (3) by contactingwith sodium, or potassium hydroxide to form the salts of said acids, and(6) returning said salts of said acids to the electroplating bath, or toother desired use.

Sodium and potassium 'cyanides are used in large quantities in theelectroplating of the metals copper, nickel, zinc, silver, gold andcadmium, and these highly toxic substances, together with complexcyanides of these metals, frequently go to the sewer as rinse washes,vitiated baths and accidental spills. It is imperative that the cyanidevalues be completely removed from plant effluent in order to avoidstream pollution, and it is equally important that the metals beremoved, as these are objectionable pollutants also.

Economically, it is highly desirable to effect the removal in a mannersuch that the cyanides and metals can be recycled to the platingoperation with a minimum of intervening processing.

It is an object of this invention to accomplish complete removal of bothcyanides and metals, followed by their sub sequent recovery for reuse ina form adapted to electroplating utilization. However, it will beunderstood that this process can also be used where the objective issolely removal per se to safeguard against pollution.

The invention will be explained with particular reference to therecovery of copper and cyanide values from a copper electroplatingoperation, wherein sodium cyanide is utilized as a plating bathingredient. It will be understood that the other metals for which myinvention is effective, i.e., nickel, zinc, silver, gold and cadmium,behave in an analogous manner to copper and, consequently, can behandled in the same general way.

The rinse water from a copper plating operation contains, in relativelydilute concentrations, sodium cyanide and the sodium salts of thecomplex cyanides of copper, e.g., NaCu(CN) or Na Cu(CN) It is necessaryto convert any sodium cyanide to a copper complex cyanide by addition ofcopper ions because, during the ion exchange process, the salts areconverted to their corresponding acids, and the hydrogen cyanide,converted from sodium cyanide, will not be held, except to a verylimited extent, by a weak base resin. However, the acids correspondingto the complex cyanides of copper are held by a weak base resin. Ashereinafter described, cuprous cyanide is especially preferred; however,cuprous oxide can be used instead, or in mixture with cuprous cyanide.

The conversion of the free sodium cyanide to sodium cuprocyanide can beachieved conveniently by passing the solution containing the sodiumcyanide, i.e., the bath or other solution which it is desired to treat,through a column-packed segarate pills or pellets fonned by compressingcuprous cyam e.

The ion exchange system utilized is composed of a mixture of thehydrogen form of an acid resin and the free base form of a weak baseresin. As is customary, the two resins should be of sufficientlydifferent densities to permit their segregation, during backwash, onefrom another-into separate layers, so that each can be regenerated by aseparate regenerant when the bed becomes exhausted.

A weak acid resin of the carboxylic type is particularly desirable inmany applications, because of its high capacity and also because it canbe regenerated effectively with a weak acid solution. This particulartype of resin, in the desired hydrogen form, is regarded as a slowresin, because it ionizes slowly. The reaction rate in the mixed bed canbe increased if .a strong acid resin, e.g., a sulfonic acid resin, issubstituted forpart of the weak acid resin. The specific gravities ofthe two cation resins should be somewhat alike to permit their jointseparation from the weak base resin by backwashmg.

If desired, the acid resin can be of strong acid form exclusively;however, this is normally not preferred because of the excessive amountof regenerant which must be employed, and this has to be disposed ofultimately by neutralization, which is both expensive and troublesome.Phosphonic type resins are intermediate in strength between carboxylicand sulfonic types, and can be utilized if desired, although these donot regenerate as efficiently as the weak acid resins such as thecarboxylic type, which are preferred in this invention.

After copper salt treatment, the rinse water is passed into the mixedbed, where the-sodium salts of the cyanide complexes are converted, to aslight degree, to their corresponding acids by the acid form resin.These acids are immediately removed from the solution by the weak baseresin. Removal of the acids disturbs the reaction equilibrium,permitting more acids to be generated, and these are progressivelyremoved as formed. After a relatively short time, all of the complexcyanides, constituting the total of cyanide values, are removed from therinse waters. The complex cyanides which are isolated on the weak baseresins are recovered for recycle to the process by regeneration of thisresin with a suitable base, which can be sodium hydroxide solution orthe even lower cost ammonium hydroxide, if presenceof ammonium ions isnot objectionable from the standpoint of ultimate disposition of theeffluent. The regeneration effluent can be relatively concentrated sothat the mixed sodium salts of the copper cyanide complexes can berecycled directly, without concentration, to the electroplating bath torestore copper and cyanide values to the process. The several reactionscan be represented by the following equations:

2. Resin-COOH NaCu(CN) COONa 4. Resin-NR HCu(CN) NaOH NaCu(CN) Resin-NR, H O

The process is completely effective in the removal of cyanide valuesfrom rinse waters containing highly variable HCu(CN) Resinamounts ofcyanide value. Usually, the first rinse water contains relatively highcyanide concentrations, whereas dilution occurs as washing proceeds. Myprocess is not sensitive to such concentration variations and is easilycapable of removing concentrations of copper complex cyanide as high as3,000 ppm or more, so that the effluent approaches 1 ppm cyanide.

Where single metals are plated, only the ions of the metal involved arerequired to convert .to the complex metal cyanide forms indicated byEquations (1). For brass plating, the addition of a mixture of copperand zinc ions is desirable. For best utilization of the exchange resins,it is preferred that the metals be added as their cyanides, or oxides.If other salts are added, the exchange capacity of the anion resin isutilized by the acid of the foreign anion, and this foreign anion couldthen be returned to the plating system when the exchanger isregenerated.

It is, of course, possible to operate the exchanger as a chromatographiccolumn during the loading or the regeneration, to thereby displace theforeign anions such as sulfate or chloride, which avoids contaminationof the bath, and thereafter route the cyanide values to the bath.However, this is a complication which is avoided by selection of thecyanide as the preliminary treatment salt.

The process of this invention is entirely applicable to potassiumcyanide as well as sodium cyanide, the former being particularlypreferred as an electroplating bath ingredient for high speed platingand certain surface effects.

At the end of any run, and before regeneration, it is desirable to rinsethe entire mixed bed with a base, such as a water solution of NaOH, NaCO or Nl-l O1-l, in order to eliminate any cyanide hazard arising fromresidual occluded cyanidecontaining solution on or within the resinparticles. This concurrently regenerates the base resin.

Following this, the bed is backwashed to separate the acid resin and thebase resin into individual layers, and the acid resin is regenerated bycontacting it alone with an acid solution. Thereafter, after rinsingresidual regenerating acid and base from the two resin layers, the tworesins are remixed by air, restoring the system to its originalcondition for reuse in cyanide removal service.

Since the efficacy of the mixed resin bed is especially dependent uponintimate contact between well-mixed particles of both acid and baseresins, it is desirable to add a small amount of bentonite suspension tothe fresh resin at start-up in order to forestall resin particleclumping, all as taught in U.S. Pat. No. 3,262,891. It has been foundthat a single treatment with bentonite at the outset is usually all thatis necessary, and that retreatment is required only when fresh resin isadded to the system.

Weak carboxylic acids can include those taught in U.S. Pat. No.2,340,111, such as Rohm & l-lass Co. lRC-SO, which is a copolymer madefrom methacrylic acid and divinylbenzene. Another such resin is lRC-84,which is a polymer made from crosslinked acrylic acid.

The weak base resin can be IRA-93; however, lR-68, having thecomposition taught in U.S. Pat. Nos. 2,591,574, and 2,675,359, can alsobe used.

EXAMPLE Approximately 105 ml. of hydrogen form CC-3 resin, a weakcarboxylic acid type marketed by the Diamond Shamrock Co., was treatedsuccessively three times by contacting in a beaker with three separatevolumes of 100 ml. each 5 percent l-lCl. The resin was then transferredto a 25 mm. ID. glass column and further treated with 100 ml. of 5percent l-lCl.

The resin was then rinsed with 500 ml. of distilled water until thecolumn effluent was almost neutral to methyl red indicator. Thethoroughly regenerated and rinsed resin was then removed from thecolumn. 100 ml. of the regenerated and rinsed CC-3 resin were then mixedwith 100 ml. of Rohm & Haas Co. IRA-93 weak base resin, which was used,without pretreatment, in the free base form.

A very small amount (weighing less than 0.5 gm) of ground Wyomingbentonite was added to destroy electrostatic charges in the resins,which were well mixed by stirring with a spatula. The mixedresin-bentonite was then transferred to a 25 mm ID. glass column,forming a bed measuring 41.7 cm. in depth.

A solution approximately equivalent to rinse water from a copper cyanideelectroplating process was prepared by diluting plating bath solution to0.1 its original concentration. The undiluted plating bath contained7.44 oz. Cu/gal. and 1.67 oz. NaCN/gal.

One-thousand ml. of the diluted bath were run downwardly through themixed resin bed during a period of mins., i.e., at a rate of about 7.1ml./min., until there was a breakthrough of copper. Breakthrough wassignaled by the development of the characteristic blue color of copperwhen a sample of effluent was mixed with ordinary household bleach,containing about 4 percent NaOCl. At breakthrough, the bed heightmeasured 48.5 cm. After rinsing with 2,000 ml. of water in 55 minutes,the bed height measured 49.2 cm.

The mixed bed was then treated with ml. of 4 percent NaOH solutionduring a 33 minute period, which was followed by a 35 minute rinse with320 ml. of distilled water, the rinse being collected in the same vesselwith the NaOH wash. The resin bed height at this point measured 51.6 cm.

A material balance showed that there was 5.58 gm. Cu in the 1,000 mi. ofdiluted bath fed to the resin, which calculates to 0.879milliequivalents of Cu/ml. of weak base resin. The theoretical capacityof the resin was 1.4 milliequivalent/ml. Analysis of the 4 percent NaOHsolution plus rinse showed that a total of 23 milliequivalents of copperwas recovered, i.e., 0.23 milliequivalent per ml. of weak base resin.Further tests employing 10 percent NaOH as copper cyanide complexdisengaging agent showed substantially complete removal of all copperfrom the resin.

What is claimed is:

l. The method of removing cyanide values from an aqueous processsolution containing compounds of the group consisting of sodium cyanide,potassium cyanide, and the sodium, and potassium salts of the complexcyanides of the metals copper, nickel, zinc, silver, gold, and cadmiumcomprising, in sequence, the steps: (1) adding sufficient ions of thegroup copper, nickel, zinc, silver, gold and cadmium to said processsolution to convert the sodium and potassium cyanide content thereof tothe sodium and potassium salts of the complex cyanides of copper,nickel, zinc, silver, gold and cadmium, (2) passing said processsolution from step (1) through an ion exchange bed composed of a mixtureof the hydrogen form of a cation exchanger selected from the groupconsisting of carboxylic, phosphonic, and sulfonic ion exchange resinsand mixtures thereof, and the free base form of a weak base resin anionexchanger to convert a portion of said sodium and potassium salts ofsaid complex cyanides of copper, nickel, zinc, silver, gold and cadmiumto their corresponding acids, (3) removing said corresponding acidsprogressively, as formed, by combination with said weak base resin untilsubstantially all cyanide values are removed from said process solution,and (4) withdrawing the process solution from the ion exchange bed.

2. The method of removing cyanide values from an aqueous processsolution according to claim I wherein said ion exchange bed is a mixtureof the hydrogen form of a carboxyl ic ion exchange resin and the freebase form of a weak base resin.

3. The method of removing cyanide values from an aqueous processsolution according to claim 1 wherein said ion exchange bed is a mixtureof the hydrogen form of a phosphonic ion exchange resin and the freebase fonn of a weak base resin.

4. The method of removing cyanide values from an aqueous processsolution according to claim 1 in which the cation exchange portion ofthe mixed bed is composed of a mixture of carboxylic and sulfonicresins.

5. The method of removing cyanide values from an aqueous processsolution according to claim 1 wherein said acids are periodicallyremoved from the weak base resin of step (3) by contacting with sodiumor potassium hydroxide to form the salts of said acids.

6. The method of removing cyanide values from an aqueous processsolution according to claim 5 wherein said salts of said acids arereturned to said electroplating bath or other desired use.

* k a e

2. The method of removing cyanide values from an aqueous processsolution according to claim 1 wherein said ion exchange bed is a mixtureof the hydrogen form of a carboxylic ion exchange resin and the freebase form of a weak base resin.
 3. The method of removing cyanide valuesfrom an aqueous process solution according to claim 1 wherein said ionexchange bed is a mixture of the hydrogen form of a phosphonic ionexchange resin and the free base form of a weak base resin.
 4. Themethod of removing cyanide values from an aqueous process solutionaccording to claim 1 in which the cation exchange portion of the mixedbed is composed of a mixture of carboxylic and sulfonic resins.
 5. Themethod of removing cyanide values from an aqueous process solutionaccording to claim 1 wherein said acids are periodically removed fromthe weak base resin of step (3) by contacting with sodium or potassiumhydroxide to form the salts of said acids.
 6. The method of removingcyanide values from an aqueous process solution according to claim 5wherein said salts of said acids are returned to said electroplatingbath or other desired use.